This invention in general relates to the field of measurement of optical elements. More specifically, this invention relates to a process for measurement of at least one parameter of an optical element which has front and back surfaces, and relates to the use of an auxiliary member to reduce reflections from the back surface of the element in order to simplify measuring the desired parameter of the front surface.
Throughout the process for fabricating optical systems from the simple to the more complex, it is frequently necessary to determine if, and how well, optical surfaces or elements conform to their designer""s stated requirements. Not only does the performance of optical systems in final form need to be verified but various parameters of their components need to undergo intermediate testing for conformance with their specifications prior to final assembly as a system. Indeed, even the tools of fabrication, especially molds for the formation of plastic or glass lens elements, need to be tested for compliance with design specifications.
Some of the most frequently encountered measurements that need to be made are radius of curvature of surfaces in either convex or concave form, thickness, power, and various focal lengths. Classically, radii of curvature is measured through the use of a hand-held instrument called a spherometer, which measures the sagittal height (sag) of the surface over a known diameter and then displays the radius of curvature on a dial or other visual display after an internal calculation that relates radius to sag height and the known diameter. However, such devices are prone to relatively large errors because sag heights are usually small dimensions that are difficult to accurately measure mechanically.
A more accurate technique for radii measurement involves the use of an auto-collimating microscope in an arrangement referred to as a radiusscope. Here, one first focuses on the surface to be measured and then on the center of curvature of the surface where a reticule image has been formed back on itself by reflection from the test surface. The positions of the microscope are recorded, and the difference between them represents the radius of curvature to limits of accuracy which depend on the preciseness of the length measurements and the ability of the operator to accurately focus on the reference points.
Where the spherometer suffers from problems of precision, the use of the radiusscope, which can be accurate to microns if care is taken, is time consuming and dependent on operator skill and experience.
The thickness of an optical element is more or less important depending on its assigned role in a particular design and can be critical where the design relies heavily on its precision for aberration control or the like. Thickness obviously can be measured directly by mechanical means which may also be automated, but there is always the danger of damaging part surfaces with mechanical approaches.
Power and focal length are always of interest and can be calculated from classical lens makers formulae having knowledge of the various numerical values required as, for example, index of refraction, radii, and thickness.
Recently, more highly automated optical measurement systems have been developed. For example, U.S. Pat. Nos. 5,280,336; 5,416,574; and 5,661,816 (all of which are assigned to the same assignee as the present application) describe systems for the automatic and rapid measurement of parameters of optical elements. These systems are computer-controlled and, once properly programmed by an operator, can measure parameters of a large number of optical elements without further human intervention and without requiring any visual observations to be made by the operator.
Most systems for measuring parameters of optical elements, including the automated systems mentioned above, rely upon reflection of light from the surface whose parameters are being measured (hereinafter usually called the xe2x80x9cfront surfacexe2x80x9d since it is normally the surface which faces the measuring system during the parameter measurement process). In most cases, optical elements have a pair of curved optical surfaces on opposed sides of the element, as for example the two curved surfaces on opposed sides of a conventional convex lens. Thus, during the measurement process, there is disposed adjacent the measuring system not only the front surface which it is desired to measure, but also a xe2x80x9crearxe2x80x9d surface on the opposed side of the optical element. Typically, this rear surface will also produce reflections which are comparable in intensity with those produced by the front surface, and can easily be confused with the front surface reflections, thus producing erroneous results. These rear surface reflections can be a particular problem in automated systems such as those mentioned above; when measurements are taken visually, an experienced operator may be able to separate the front and rear surface reflections by visual observation, but it is much less easy to separate the two series of reflections when analysis of the optical data is being effected by a computer program.
Numerous attempts have been made to eliminate rear surface reflections during optical parameter measurement processes, or at least to reduce these rear surface reflections to a point at which it is easy to distinguish them from the desired front surface reflections. One approach to the problem is to render the rear surface a dark color, for example with a colored marker such as those sold under the trade name Magic Marker. For example, applying black marker to the rear surface (the exact color is immaterial, a dark blue, green or red being equally effective) will greatly reduce the intensity of the rear surface reflections. However, such colored markers typically cannot be completely removed from the rear surface, even with the use of organic solvents, and thus leave a permanent residue (stain) on the rear surface, and the stained optical element usually cannot be sold after the testing. The resultant scrapping of the tested element greatly increases the cost of testing, and obviously renders this method useless in situations where it is desired to test all production units of an optical element.
An alternative approach to reducing rear surface reflections is to cover the rear surface with a material which has a refractive index close to that of the material on which this rear surface is formed; the covering material can be colorless so that it does not stain the optical element. As is well known to those skilled in optics, the intensity of reflections from a boundary between two different materials is a function of the difference between the refractive index of the materials which meet at the boundary, so that by making this difference small the intensity of reflections can be reduced to a point at which they will not interfere with measurements based upon the front surface reflections. For example, Rotlex Ltd, Rotem Industrial Park, D. N. Arava, Israel sells a water-soluble grease, having a consistency similar to that of petroleum jelly, as a covering material for use on optical elements. While this material does not stain the element, is does adhere tenaciously to most optical element surfaces and does not have significant mechanical cohesion, so that it must be wiped from the surface after testing is complete and may require washing of the surface with water or other solvent for complete removal.
From the foregoing, it will be seen that the prior art solutions to suppressing rear surface reflections are far from satisfactory. Furthermore, these prior art solutions are impracticable for use with automated measuring apparatus such as those described in the aforementioned patents. For example, the preferred apparatus of U.S. Pat. No. 5,416,574 is designed to carry out a measurement in approximately three seconds. If such an apparatus were used to measure the parameters of a large batch of optical elements using either a colored marker or a jelly-like material to suppress rear surface reflections, it would be difficult if not impossible to arrange for removal of the material applied to the rear surface at the throughput which the measuring apparatus is capable of achieving.
It is an object of the present invention to provide a process for measuring at least one optical parameter which enables the suppression of rear surface reflections without requiring the application of either a colored or semi-solid material to the rear surface.
It is also an object of the present invention to provide such a process in which essentially no residue is left on the optical element, so enabling the optical element to be sold commercially and thus avoiding the expense of scrapping the tested element.
It is also an object of the present invention to provide such a process which is well-adapted for use with high speed automated testing equipment in that the process does not require a lengthy step for removal of material applied to the rear surface of the optical element.
Other objects of the invention will be apparent and will appear hereinafter in the following detailed description when read in connection with the drawing.
This invention provides a process for measuring at least one parameter of an optical element having first and second surfaces on opposed sides thereof. The process of the invention uses a reflection-reducing member, this member being solid and having substantial mechanical integrity but being resilient and elastically deformable so as to enable at least one of its surfaces to conform to a surface with which it is placed in contact. In the present process, the reflection-reducing member is placed in contact with the second surface of the optical element, whereby a surface of the reflection-reducing member conforms to said second surface. At least one parameter of the first surface of the optical element is measured, and the reflection-reducing member is then removed from the second surface of the optical element leaving this second surface substantially free from any residue from the reflection-reducing member.
This invention also provides, in combination, an optical apparatus and a reflection-reducing member. The optical apparatus comprises means for supporting and positioning an optical element in a substantially fixed position on the optical apparatus with at least a second surface of the optical element exposed, and means for measuring at least one parameter of a first surface of the optical element. The reflection-reducing member is solid and has substantial mechanical integrity but is resilient and elastically deformable so as to enable at least one surface of this member to conform to a surface with which it is placed in contact.